Lignin Is Amorphous And Highly Complex

Published Date: 02 Nov 2017

The bioconversion of lignocellulosics to ethanol consists of two main processes: hydrolysis of lignocellulosic carbohydrate to fermentable reducing sugars, and fermentation of the sugars to ethanol (Figure 5). The hydrolysis is usually catalyzed by cellulase enzymes, and the fermentation is carried out by yeasts or bacteria. The factors that have been identified to affect the hydrolysis of cellulose include porosity (accessible surface area) of the waste materials, cellulose fiber crystallinity, and lignin and hemicellulose content (Mosier et al. 2005; Margeot et al. 2009; Alvira et al. 2010). The presence of lignin and hemicellulose in lignocellulosic materials make the access of cellulase enzymes difficult, thus reducing the efficiency of the hydrolysis (Himmel et al. 2007). Pretreatment of lignocellulosic biomass prior to hydrolysis can significantly improve the hydrolysis efficiency by removal of lignin and hemicellulose, reduction of cellulose crystallinity, and increase of porosity (McMillan 1994; Palmqvist and Hahn-Hagerdal 2000a, b; Sun and Cheng 2002; Mosier, Wyman, Dale, et al. 2005; Kumar, Barette, Delwiche, et al. 2009).

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Figure 5: Schematic representation of process for bioethanol production from lignocellulosic biomass.

4.1. Pretreatment of lignocellulosic biomass

Lignocellulosic biomass, the most abundant and low-cost biomass worldover are the raw materials for production of fuel ethanol. For transformation of lignocelluosics into bioethanol, the steps involved are procurement of lignocelluosics, pretreatment, hydrolysis to derive sugars their fermentation bioethanol and its dehydration. Lignocellulosics are composed of heterogeneous complex of carbohydrate polymers (cellulose, hemicelluloses and lignin). Cellulose (40-60% by wt) consists of high molecular weight polymers of glucose rigidly held together as bundles of fibers. Hemicellulose (20-40% by wt) is shorter polymers of various sugars that bind cellulose bundles together. Lignin (10- 30% by wt) consists of a tri-dimensional polymer of propyl-phenol that is imbedded in and bound to hemicellulose to provide rigidity (Figure 6).

Figure 6: Schematic of the role of pretreatment in the conversion of biomass to fuel.

Pretreatment of lignocellulosics aims to decrease crystallinity of cellulose, increase biomass surface area, remove hemicellulose, and break the lignin barrior. Pretreatment makes cellulose more accessible to hydrolytic enzymes to facilitate conversion of carbohydrate polymers into fermentable sugars in a rapid way with the concomitant more yield. Pretreatments include physical, chemical and thermal methods, and their combinations. Pretreatment is one of the most expensive processing steps for production of sugars from biomass. Since many lignocellulosics have different physicochemical characteristics, it is necessary to deploy suitable pretreatment technology based on their properties.

4.2 Goals of Pretreatment

The beneficial effects of pretreatment of lignocellulosic materials have been recognized for a long time.The goal of the pretreatment process is to remove lignin and hemicellulose, reduce the crystallinity of cellulose, and increase the porosity of the lignocellulosic materials. Pretreatment must meet the following requirements: (1) improve the formation of sugars or the ability to subsequently form sugars by hydrolysis, (2) avoid the degradation or loss of carbohydrate, (3) avoid the formation of byproducts that are inhibitory to the subsequent hydrolysis and fermentation processes, and (4) be cost-effective. Pretreatment methods can be roughly divided into different categories: physical (milling and grinding), physicochemical(steam pretreatment/autohydrolysis, hydrothermolysis, and wet oxidation), chemical (alkali, dilute acid, oxidizing agents, and organic solvents), biological, electrical, or a combination of these. Various physical, physico- chemical, chemical, and biological processes have been used for pretreatment of lignocellulosic materials (Table 2). Among them Acid hydrolysis technique has been found one of the most promising pretreatment. The main objective of the acid pretreatments is to solubilize the hemicellulosic fraction of the biomass and to make the cellulose more accessible to enzymes. This type of pretreatments can be performed with concentrated or diluted acid but utilization of concentrated acid is less attractive for ethanol production due to the formation of inhibiting compounds. Concentrated acids such as H2SO4 and HCl have been used to treat lignocellulosic materials. Although they are powerful agents for cellulose hydrolysis, concentrated acids are toxic, corrosive and hazardous and require reactors that are resistant to corrosion. In addition, the concentrated acid must be recovered after hydrolysis to make the process economically feasible (Sivers and Zacchi, 1995). Diluted acid pretreatment appears as more favourable method for industrial applications and have been studied for pretreating wide range of lignocellulosic biomass. Different types of reactors such as percolation, plug flow, shrinking-bed, batch and countercurrent reactors have been applied for pretreatment of lignocellulosic materials (Taherzadeh and Karimi, 2008). Dilute acid hydrolysis has been successfully developed for pretreatment of lignocellulosic materials. The dilute sulfuric acid pretreatment can achieve high reaction rates and significantly improve cellulose hydrolysis (Esteghlalian et al., 1997). At moderate temperature, direct saccharification suffered from low yields because of sugar decomposition. High temperature in dilute acid treatment is favorable for cellulose hydrolysis (McMillan, 1994). Recently developed dilute acid hydrolysis processes use less severe conditions and achieve high xylan to xylose conversion yields. Achieving high xylan to xylose conversion yields is necessary to achieve favorable overall process economics because xylan accounts for up to a third of the total carbohydrate in many lignocellulosic materials (Hinman et al., 1992).It can be performed at high temperature (e.g. 180˚C) during a short period of time; or at lower temperature (e.g. 120˚C) for longer retention time (30–90min). It presents the advantage of solubilizing hemicellulose, mainly xylan, but also converting solubilized hemicellulose to fermentable sugars. Nevertheless, depending on the process temperature, some sugar degradation compounds such as furfural and HMF and aromatic lignin degradation compounds are detected, and affect the microorganism metabolism in the fermentation step (Saha et al., 2005). Anyhow, this pretreatment generates lower degradation products than concentrated acid pretreatments. High hydrolysis yields have been reported when pretreating lignocellulosic materials with diluted H2SO4 which is the most studied acid. Hydrochloric acid, phosphoric acid and nitric acid have also been tested (Mosier et al., 2005a). Saccharification yield as high as 74% was shown when wheat straw was subjected to 0.75% v/v of H2SO4 at 121˚C for 1 h (Saha et al., 2005). Olive tree biomass was pretreated with 1.4% H2SO4 at 210˚C resulting in 76.5% of hydrolysis yields (Cara et al., 2008). Recently, ethanol yield as high as 0.47 g/g glucose was achieved in fermentation tests with cashew apple bagasse pretreated with diluted H2SO4 at 121˚C for 15 min (Rocha et al., 2009). Organic acids such as fumaric or maleic acids are appearing as alternatives to enhance cellulose hydrolysis for ethanol production. In this context, both acids were compared with sulfuric acid in terms of hydrolysis yields from wheat straw and formation of sugar degradation compounds during pretreatment. Results showed that organic acids can pretreat wheat straw with high efficiency although fumaric acid was less effective than maleic acid. Furthermore, less amount of furfural was formed in the maleic and fumaric acid pretreatments than with sulfuric acid (Kootstra et al., 2009).